Fuel evaporative gases generated within a fuel tank become a cause of air pollution. Thus, a vehicle having an engine loaded thereon is generally loaded with a fuel evaporative gas treatment device for suppressing the emission of the fuel evaporative gases into the atmosphere. The fuel evaporative gas treatment device is configured, for example, to connect the fuel tank and an intake system of the engine by a purge conduit equipped with a canister so as to adsorb the fuel evaporative gases, which have been generated within the fuel tank, to activated carbon within the canister once, and introduce fuel adsorbed to the activated carbon into the intake system of the engine in response to the intake negative pressure of the engine, thereby burning the fuel together with fresh air.
In recent years, vehicles having a traveling motor along with an engine, such as plug-in hybrid electric vehicles (PHEV) and hybrid electric vehicles (HEV), have found practical use. With such vehicles equipped with the traveling motor, the period of engine stoppage, namely, the period during which fuel cannot be introduced from the canister into the intake system of the engine, may continue relatively long. Thus, a so-called closed type fuel evaporative emission control apparatus, in which a containment valve is provided between the fuel tank and the canister, and this containment valve remains closed during the period of the engine stopping, has been developed. Among the closed type fuel evaporative emission control apparatuses is one, for example, in which an on-off valve is provided near an inlet of the canister and, when the on-off valve is closed, fuel evaporative gases are not introduced into the canister, but directly introduced into the intake system of the engine.
In the fuel evaporative emission control apparatus of the above configuration, leaks of fuel evaporative gases in case of trouble, if any, would directly lead to air pollution. In the United States, etc., therefore, it is legally obligated to detect leaks of fuel evaporative gases. Under the laws and regulations of the United States, in particular, it is obligated to perform on-board diagnosis (OBD) for detecting such leaks of fuel evaporative gases and, if the leaks are detected, to inform the driver, for example, by lighting a warning lamp. Needless to say, the closed type fuel evaporative emission control apparatus is also required to detect leaks similarly.
To fulfill such requirements, there is, for example, an apparatus available which is designed to detect the presence or absence of leaks of fuel evaporative gases based on a difference between the pressure inside the fuel tank and the atmospheric pressure. Concretely, a forced on-off valve is provided midway through a vapor passage connecting the canister and the fuel tank together, the forced on-off valve is closed except during refueling, and the internal pressure of the fuel tank at the cold start of the engine or in the closed state of the tank with the forced on-off valve being closed at the start is measured. Only on condition that the absolute value of the difference between the internal pressure of the tank and the atmospheric pressure is equal to or higher than a predetermined determination value showing that the airtightness of the fuel tank is fully ensured, it is determined that there is no abnormality, such as leakage, in the fuel tank (See Patent Document 1).